Fiber optics have provided a substantial increase in the volume of data that networks can carry. Synchronous Optical Network (SONET) is a standard that defines telecommunication transmissions over fiber optics. SONET defines a number of different data rates for different levels of service. A SONET network can carry from about 52 Mega bits per second to about 10 Gigabits per second over a single optical fiber. In comparison, a typical analog modem operating at maximum efficiency can achieve a mere 56 Kilobits per second. At the 10 Gigabit SONET rate, a single optical fiber can carry enough data to handle well over 100,000 simultaneous voice calls. SONET networks are likely to carry even larger volumes of data in the future.
SONET is generally used as a “trunk” or “backbone” network due to the large volume of data that SONET can handle. That is, most network customers these days do not need 10 Gigabits of bandwidth per second and do not want to pay for unneeded bandwidth. In which case, customers are generally provided with lower data rate services like analog phone lines, Digital Subscriber Lines (DSLs), digital cable lines, Integrated Service Digital Network (ISDN) lines, T1 lines, etc. These lower rate services typically connect customers in a given region to a local “central office.” Central offices, in turn, are connected by trunk, or backbone, lines, like SONET lines.
Many smaller data streams from the lower rate services are combined in a SONET line to travel among central offices. A data stream that goes into a SONET line at one central office will be packaged into SONET frames for transmission. At the destination office, the data stream will be recovered, or rebuilt, from the data in the SONET frames.
In addition to the user data, a SONET line is likely to carry a considerable amount of control data. That is, a central office that is packaging a data stream needs to tell the receiving central office various things about the data stream, such as what format the data stream uses and what level of service the data stream uses from potentially many available formats and levels of service.
One approach to communicating control data among central offices is to define an additional control channel to carry the control data. An additional control channel, however, generally uses a certain amount of bandwidth that could otherwise be used to transmit user data. Since users generally pay for transmitting user data and not for transmitting control data, using bandwidth for a control channel tends to reduce revenue.